1. Field of the Invention
The present invention relates to a thermal mass flowmeter for measuring a mass flow rate of a fluid flowing through a capillary tube, based on a temperature distribution in a flow direction of the fluid.
2. Description of the Related Art
With reference to FIGS. 2A and 2B, a conventional thermal mass flowmeter will be described. FIGS. 2A and 2B illustrate one example of the conventional thermal mass flowmeter, wherein FIG. 2A is a sectional view showing a measurement site, and FIG. 2B is a graph showing a temperature distribution on a surface of a capillary tube. In FIG. 2B, the vertical axis represents a surface temperature of the capillary tube, and the horizontal axis represents a position of the capillary tube in a flow direction of a fluid. The dashed curve represents a temperature distribution in a state when a fluid in the capillary tube is in a quiescent state (i.e., in a non-flowing state), and the solid curve represents a temperature distribution in a state when the fluid is flowing through the capillary tube (i.e., in a flowing state).
In the measurement site, a heating unit 12 is disposed in contact with an outer peripheral surface of the capillary tube 11. A pair of temperature-sensing units 14, 16 for measuring a surface temperature of the capillary tube 11 are also disposed in contact with the outer peripheral surface of the capillary tube 11 at respective positions equally distant from the heating unit 12 toward a downstream side and an upstream side of the capillary tube 11 along the flow direction. This thermal mass flowmeter employs a flow rate-measuring chip 18 prepared by incorporating the heating unit 12 and the temperature-sensing units 14, 16 into a single substrate using MEMS (Micro Electro Mechanical System) fabrication techniques, wherein the flow rate-measuring chip 18 is mounted to the capillary tube 11 to measure a flow rate of a fluid flowing through the capillary tube 11 (see, for example, U.S. Pat. No. 6,813,944).
In the thermal mass flowmeter, the surface temperature of the capillary tube 11 is measured using the pair of temperature-sensing units 14, 16 disposed in spaced apart relation to each other by a given distance, while locally heating the fluid in the capillary tube 11 up to a given temperature by the heating unit 12 (i.e., while locally heating a wall of the capillary tube up to a given temperature by the heating unit 12). When the fluid is in the non-flowing state, the surface temperature of the capillary tube 11 exhibits a bilaterally (streamwisely)-symmetrical temperature distribution profile having a peak at a position of the heating unit 12, as indicated by the dashed curve in FIG. 2B, and therefore no difference exists between respective measured (i.e., sensed) temperatures of the temperature-sensing units 14, 16 disposed on the downstream and upstream sides relative to the heating unit 12
When the fluid starts flowing through the capillary tube 11, the surface temperature distribution profile of the capillary tube 11 is displaced toward the downstream side in its entirety, as indicated by the solid curve in FIG. 2B, and thereby a certain difference occurs between respective sensed temperatures of the temperature-sensing units 14, 16. More specifically, along with an increase in flow rate of the fluid flowing through the capillary tube 11, the surface temperature distribution profile of the capillary tube 11 is displaced a greater distance toward the downstream side. During this process, as long as the peak of the surface temperature distribution profile is located between the heating unit 12 and the downstream-side temperature-sensing unit 14, the difference between the respective sensed temperatures of the temperature-sensing units 14, 16 becomes larger as the peak position comes closer to the downstream-side temperature-sensing unit 14. That is, on the condition that the peak of the surface temperature distribution profile is located between the heating unit 12 and the downstream-side temperature-sensing unit 14, there exists a certain correlation between the flow rate of the fluid flowing through the capillary tube 11 and the difference between the respective sensed temperatures of the temperature-sensing units 14, 16. This correlation can be figured out in advance to allow the flow rate of the fluid in the capillary tube 11 to be calculated from the difference between the respective sensed temperatures of the temperature-sensing units 14, 16
A low-cost approach to incorporating the heating unit 12 and the temperature-sensing units 14, 16 into a single substrate using MEMS fabrication techniques is hardly achieved due to a problem about required fabrication facilities, etc.
Therefore, there has been proposed a technique of mounting a temperature-sensing unit and a heating unit to an outer peripheral surface of a capillary tube separately so as to make up a thermal mass flowmeter without using MEMS fabrication techniques.
In the type of thermal mass flowmeter having the configuration where a temperature-sensing unit and a heating unit are mounted to an outer peripheral surface of a capillary tube separately, a diode unit is often used as the temperature-sensing unit. The reason is that the diode unit can be driven at a constant voltage to obtain a sensitivity of about 80,000 ppm/° C. which is the highest value among conventional temperature-sensing units. The diode unit is also often used as the heating unit. Typically, in commercially-available diode units, for the purpose of protecting a diode element, a resin is molded around the diode element to allow the diode element to be disposed in a central region of the molded resin. Thus, despite a requirement that a temperature-sensing unit of a thermal mass flowmeter must measure only a temperature change of a target fluid (i.e., a temperature change on an outer peripheral surface of a capillary tube), the diode unit having such a structure will additionally measure an unintended temperature change in its surroundings in such a manner as to be superimposed on a desired sensing signal. Consequently, the unintended temperature change becomes a noise component of the sensing signal to cause deterioration in SN ratio (Signal-to-Noise Ratio) (hereinafter referred to as “S/N”).
Moreover, the molded resin of the diode unit is made of an epoxy resin which is not a thermal conductor. Thus, the diode unit used as the temperature-sensing unit or the heating unit will have a delay in thermal response, resulting in a poor response of the thermal mass flowmeter.